The 6M was used in all 7M blocks until they started phasing it out in '89. I've no idea why it was sprinkled in throughout '89 and I think maybe in some '90 year engines. The 7M is much beefier and better balanced, there are reasons one may choose to go with either one.
The only thing the N/A block saw before the GTE did were the beefier block castings (extra and stronger ribs and I think some heavier bosses and minor increases in outer wall thickness). These started showing up in '88 on the GE but not on the GTE until mid '89 or so, if I remember correctly.
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